An inverter for feeding electric energy from a DC generator into an AC power grid is known from EP 2 023 475 A1. Here, the DC/DC converter supplies at least two bipolar output DC voltages that are additively superimposed between the input lines of the inverter with regard to an earthed center point. Particularly, there are resonant circuits which are each branched and comprise two partial resonance capacitances. Each of these two partial resonance capacitances is connected to two rectifier diodes at its output, and it is thus alternately electrically conductively connected to the two lines of one division of a divided DC voltage link. An inverter bridge of the inverter is connected to the divided DC voltage link. In this way, the known inverter comprises a basic converter ratio for the DC input voltage present between the input lines of the entire inverter as compared to a DC link voltage present between the input lines of the actual inverter bridge of 1:n, wherein n is the total number of the partial resonance capacitances towards which the resonant circuits are branched. In other words, n corresponds to the number of divisions of the divided DC link out of which the inverter bridge is fed. This basic converter ratio for the input DC voltage will prove to be disadvantageous when the known inverter is to be used for feeding electric energy from a photovoltaic module into an AC power grid, if the DC voltage provided by the photovoltaic module is already higher or at least as high as the peak voltage of the AC power grid. Some photovoltaic panels presently used provide such a high DC voltage that the peak voltage of an AC power grid to be fed is already exceeded by a multiple.
Not all embodiments of the inverter known from EP 2 023 475 A1 comprise a galvanic separation between the input lines of the inverter which are the input lines of its DC/DC converter, and the input lines of its inverter bridge which are the output lines of its DC/DC converter. In some embodiments, both the center point of the divided DC voltage link and one of the input lines of the DC/DC converter are grounded, resulting in a grounding extending beyond the DC voltage link.
In the embodiments of the inverter known from EP 2 023 475 A1 which comprise a galvanic separation, the center point of the divided voltage link is connected to the input lines of the inverter bridge via capacitances, and it is connected to an intermediate point between the input lines of the inverter which is—also via capacitances—connected to these input lines of the inverter. Via this additional connection, a passive AC current backflow path is provided which—due to its pure capacitive connection to the input lines of the inverter—does not remove the galvanic separation between the input lines of the entire inverter and the input lines of its inverter bridge.
In the embodiments of the inverter known from EP 2 023 475 A1 in which the input lines of the entire inverter are galvanically separated from the input lines of its inverter bridge and thus from the output lines of the entire inverter, a reference potential for the input DC voltage may be freely selected. There is, however, the danger that the current sum of the currents flowing via the resonant circuits and the passive AC current backflow path is not balanced to zero and that thus undesired, quite high compensation currents may flow via ground.
For example, common mode currents via the DC/DC converter unavoidably occur if, in a DC/DC converter whose output side is capacitively decoupled from its input side, the input side has a fixed potential with reference to ground due to grounding one of its input lines, whereas the potential of its output side with reference to ground periodically changes due to being connected to an inverter bridge which feeds into an AC power grid having a defined potential with reference to ground.
A variant of a so-called Single-Ended Primary Inductance Converter (SEPIC) with galvanic separation due to an additional capacitance in an AC current backflow path is known from EP 0 696 841 A1. Here, there also is the danger that the current sum of the currents flowing via the AC current forward path and the passive AC current backflow path are not balanced to zero and thus undesired compensation currents have to flow via earth.
There still is a need for a DC/DC converter in which compensation currents via ground can be principally avoided.